The present invention relates generally to the field of mobile wireless data communications and, more particularly, to a communications system where each communications unit is capable of determining its own geographic location e.g., from the Global Positioning System (GPS) of satellites, and forming with other units a stand-alone baseless network.
There are many types of systems where a plurality of mobile units geographically dispersed must all communicate together in a distributed, decentralized form, using a two-way flow of data over a single communications channel. Examples of mobile wireless systems are aircraft location and collision avoidance systems, telemetry and proximity radar systems, dynamic navigation, surveillance and emergency location systems, air combat monitoring systems, traffic alert systems, pilot data systems, multiple vehicle tracking and security systems, fleet control systems, mobile work force systems or location based adaptive radio control systems. In each case no static master-slave relationship exists among those mobile units. On the contrary, they are all functionally equivalent and, generally, the information coming from any one unit is made available and transmitted to all others at some point of time. Because this occurs on a common shared communications channel, as the number of units that try to access this common channel increases and they are more and more used, the time left to each of them may dramatically be impacted up to a point where messages transmitted by several units collide. Beyond this point probability of transmission collisions dramatically increases since units experiencing collisions all try to retransmit. This may reach a point where no communication becomes possible. Therefore, in such systems, it is of the utmost importance that the common communications channel implement some kind of xe2x80x9cdistributedxe2x80x9d or xe2x80x9cuniversalxe2x80x9d channel coordination or control mechanism aimed at preventing transmissions collision while it remains equally important that the common communications channel be used as efficiently as possible so as it can be shared between many users.
Also, a wireless communications environment is characterized by high error rates and by large variations on the signal propagation conditions over time and position. Parameters like distance, weather, radio shadow and multi-path propagation may dramatically affect the transmission conditions. Increasing the power of the transmitters to overcome this is limited in practice, because is mobile units means are generally portable units with limited battery capacity and safety regulations limiting maximum allowed Radio Frequency (RF) signals strength from transmitters. Thus, there is a maximum range for reliable reception of the information transmitted by distant units which practically limits the possibility that the information transmitted from any one unit can correctly be received directly by all the others. Therefore, in such systems, it is also of the utmost importance that each one of the mobile units, receiving a signal from any other unit, can also play the role of a regenerator or relay, thus eventually allowing to deliver reliable information originated from a set of units to other sets that cannot otherwise communicate directly.
Actually, methods have been proposed for enabling data communications over a fleet of mobile units. For example, U.S. Pat. No. 5,153,836, entitled xe2x80x9cUniversal Dynamic Navigation, Surveillance, Emergency Location, and Collision Avoidance System and Method,xe2x80x9d teaches how each craft must broadcast its position, identifying information and other messages on a regular basis, without having to be solicited, while preventing conflicts with any radio frequency signals possibly present in the airspace. Similarly, U.S. Pat. No. 4,835,537, entitled xe2x80x9cTelemetry Burst Collision Avoidance System,xe2x80x9d describes a system where each vehicle repeatedly broadcasts its current position and uses carrier detection to prevent transmitting when another station is active. Also, U.S. Pat. No. 4,197,538, entitled xe2x80x9cPilot""s Traffic Monitoring System,xe2x80x9d teaches how, with respect to a given aircraft, position and altitude of all other air craft are received while aircraft position and altitude is transmitted by an onboard FM transceiver multiplexer operated within a transmit-receive duty cycle and providing for transmission collisions. U.S. Pat. No. 5,907,293, entitled xe2x80x9cSystem for Displaying the Characteristics, Position, Velocity and Acceleration of Nearby Vehicles on a Moving-map,xe2x80x9d uses a GPS receiver to determine a vehicle position with respect to fixed positioned radars at blind intersections, determining other vehicle positions and broadcasting vehicle location information for vehicles lacking a means to track their own location, and acknowledges that broadcast messages could possibly collide. U.S. Pat. No. 5,872,526, entitled xe2x80x9cGPS Collision Avoidance System,xe2x80x9d describes how crafts equipped with GPS receivers are broadcasting their current location to other vehicles while vehicles not equipped with GPS are receiving location information from common ground equipment broadcasted to all vehicles and also acknowledges that transmission collisions occur.
A different approach for controlling transmissions in a distributed mobile environment is that of U.S. Pat. No. 5,367,524, entitled xe2x80x9cMethod for Sequential Data Transmission,xe2x80x9d whereby using accurate GPS clock signals as an external clock, a communications system determines a cycle time and duration of time slots such that each communications unit is allocated a time slot for data transmission. This approach eliminates the need for polling the communications units and reduces substantially data congestion, but requires some Time Division Multiplex (TDM) master controller to allocate different time slots to different units. A similar communications control approach is proposed in U.S. Pat. No. 5,587,904, entitled xe2x80x9cAir Combat Monitoring System and Methods and Apparatus Useful Therefor,xe2x80x9d where information dissemination among a plurality of aircraft is performed by employing GPS signals for synchronizing transmitters and uses TDM for communicating among the aircrafts.
Yet another approach is that of U.S. Pat. No. 5,636,123, entitled xe2x80x9cTraffic Alert and Collision Avoidance Coding System,xe2x80x9d that describes a traffic alert and collision avoidance system where the airspace is divided into a grid of volume elements, each of which is assigned a unique pseudo noise (PN) code and where a vehicle determines its position by means of a GPS receiver and places it within one of the volume elements in the airspace. The message to transmit is then generated by modulating a carrier signal with the PN code representing the volume element containing the transmitting vehicle and with the navigation message being then transmitted on a common communications channel using time-multiplexing based on a pseudo random sequence; each receiving vehicle only tracks collision avoidance signals produced by vehicles located in its own and surrounding volume elements.
Still another different approach corresponds to U.S. Pat. No. 4,380,050, entitled xe2x80x9cAircraft Location and Collision Avoidance System,xe2x80x9d which describes a system where the azimuth and range information of an aircraft with respect to a common ground reference is made available to other aircraft by transmission of a pulse at a time uniquely associated with the aircraft""s location so that the collision of the pulses transmitted by air crafts from different locations is impossible, although the method is limited to proximity detection, not enabling transmission of other relevant information, like speed, heading or craft identification. In part based on the same principle, U.S. Pat. No. 5,450,329, entitled xe2x80x9cVehicle Location Method and System,xe2x80x9d the time slot in which the data message is transmitted is uniquely assigned to each transmitting vehicle as a function of its position, determined by means of a GPS receiver or alike system, in a cartographic grid segment with respect to a reference point assigned to that grid segment. Although this method avoids transmission collisions its main drawback is that much bandwidth is wasted because there are reserved time slots for all grid segments even though not all have transmitters.
Finally, methods have been proposed aimed at controlling mobile units depending on their positions. An example is U.S. Pat. No. 5,442,805, entitled xe2x80x9cLocation-based Adaptive Radio Control,xe2x80x9d which describes a method and apparatus for a radio having adjustable operating parameters such as transmitting power, operating channel, operating band, modulation type, etc. based on the unit""s current location. In the same line, U.S. Pat. No. 5,635,940, entitled xe2x80x9cCommunication Configurator and Method for Implementing Samexe2x80x9d and U.S. Pat. No. 5,794,151, entitled xe2x80x9cFrequency Allocation for Shared Spectrum Transmitter Based on Location,xe2x80x9d describe similar methods where, depending on the location of the mobile unit, a different communications channel or frequency is configured using configuration data stored with the digital equipment. U.S. Pat. No. 5,355,511, entitled xe2x80x9cPosition Monitoring for Communicable and Incommunicable Mobile Stations,xe2x80x9d describes a system and method comprising an order station and a plurality of mobile stations. When the order station cannot communicate with a mobile station, the order station selects one mobile station which is nearest to the incommunicable mobile station and orders the selected mobile station to communicate with the incommunicable mobile station.
It is a purpose of the present invention to improve the art which can be apprehended from the above review. The present invention enables a plurality of mobile communications units, which may have to operate on large geographic distances, to communicate between them however, neither requiring a base or central station nor a universal master clock to coordinate the access to the common communications channel shared by the mobile units while maximizing the area over which communications are possible.
One aspect of the present invention is to permit bi-directional, one-to-one and one-to-many communications among a fleet of geographically spread mobile units capable of self asserting their geographic location.
The invention further allows a collision-free access of the mobile units to the common communications channel, neither requiring a central station to coordinate the exchanges nor the knowledge of a precise clock reference.
The invention makes possible that two or more mobile units, which cannot communicate directly, may communicate indirectly through other communications units belonging to the fleet of mobile units.
The invention allows transmitting the geographic locations of each communications unit to all others so as to establish a topology and determine the best paths between individual units that cannot communicate directly.
Each unit of the invention uses the topology information to tailor the transmission and reception settings so as to obtain the best possible communication between any two units.
This invention provides a system and method of forming a network, out of a plurality of mobile communications units sharing a common transmission medium without requiring a base station. Each unit is assumed to have provision for self asserting its own position and has facility for two-way communicating with the other mobile communications units. The method allows any two mobile communications units participating in the network to eventually acquire the capability of communicating even though they are not all capable of communicating directly. The initial step of the method assume that each new mobile communications unit needs to register to participate in the network. To do so, each unit not yet registered to a network broadcasts registration commands e.g., at random intervals. When such a registration command is captured by at least one other mobile communications unit registration data provided by said new mobile communications unit is recorded. Registration data includes at least the unique unit identifier of the new mobile communications unit and its current location. This is acknowledged to the new mobile communications unit which, being registered, stops broadcasting registration commands.
Then, each registered mobile communications unit establishes for the first time, or updates a polling sequence, encompassing all the mobile communications units registered to the network. A polling command is formatted, which includes the polling sequence, so that only one at a time of the registered mobile communications units is authorized to issue the polling command in the polling sequence. Finally, the polling command is issued to the next in sequence of said mobile communications units specified in said polling sequence. When the next unit captures the polling command, it is granted in turn, the permission to repeat the above steps thus moving forwards though the polling sequence of registered units.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.